Quantum Cryptography Done Over Shared Data Line

Illustration: iStockphoto

Researchers have sent quantum keys over a "lit" fiber-optic network, a step towards using quantum cryptography on the networks businesses and institutions use every day.

A group of U.K.-based research groups last week said the demonstration opens the door to more research that will make the technology more commercially viable. The researchers were from Toshiba Research Europe, BT, ADVA Optical Networking, and the U.K.'s National Physical Laboratory (NPL). 

In quantum cryptography, the keys to unlock the contents of communications are represented with photons. It starts with a laser that sends a pair of photons over a fiber-optic network. The polarization of photons—whether they’re oscillating horizontally or vertically, for example—can be detected by a receiver and read as bits, which are used to generate the same encryption key at both ends of the network connection. If an interloper attempts to intercept the keys to decrypt a message, the receiver will be able to detect a change, according to the laws of quantum mechanics. If that happens, the receiver can reject the keys and the message stays encrypted.

Until now, quantum key distribution (QKD) has been done over dark fiber, or unused optical fiber lines, which means that a separate fiber optic line is needed for transmitting other data. But dark fiber networks are not always available and are expensive. Being able to transmit quantum keys over a lit fiber network means that institutions and businesses will be able to run quantum cryptography over their existing networking infrastructure, the researchers said.

"Using techniques to filter out noise from the very weak quantum signals, we've shown that QKD can be operated on optical fibers installed in the ground and carrying conventional data signals," said Andrew Shields from Toshiba Research Europe in a statement

The National Physics Laboratory developed a series of measurements for identifying individual particles of light from the stream of photons sent over a fiber-optic line. That will allow the system to detect attempts to intercept the transmission of keys, which should improve customer confidence in quantum cryptography, said Alastair Sinclair from the National Physics Laboratory in a statement.

The test was conducted over a live BT fiber link between its research campus in Suffolk and another BT site in Ipswich, U.K. In an interview with Nature, Toshiba's Shields said the quantum key distribution was done alongside data transmitted at 40 gigabits per second, the fastest multiplexing of regular data with quantum keys to date. But he notes that implementing QKD in the "real world" is more challenging than a laboratory environment because there are environmental fluctuations that can cause data loss in fiber lines.

Another technical challenge facing widespread use of QKD is the distance keys can be sent. Light pulses sent over a fiber optic line fade, which means that key distribution can only be done at a distance of about 100 kilometers. (See Long-Distance Quantum Cryptography.) But as governments and companies seek out the most secure ways to send data, quantum cryptography could become an appealing option.

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